Angles · Grades 4–8

Angles Worksheets

Classify an angle as acute, right, obtuse, or straight, then find missing angles using complementary/supplementary pairs and the triangle angle sum.

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What it is

Angles are measured in degrees. Classify one by its size, or use special pairs and the triangle angle sum to find a missing angle.

Why it matters

Angle relationships are the backbone of geometry and lead into parallel lines, polygons, and trigonometry.

How to do it

  1. Acute: less than 90°. Right: exactly 90°. Obtuse: between 90° and 180°. Straight: exactly 180°.
  2. Complementary angles add to 90°.
  3. Supplementary angles add to 180°.
  4. The three angles of a triangle add to 180°.

Examples by level

  • Beginner
    10°?
    Find the missing angle =80°
  • Intermediate
    10°?
    Find the missing angle =80°
  • Advanced
    36°83°?
    Find the missing angle =61°

Examples are generated by the same engine as the worksheets, so they're always mathematically correct. Built to avoid repeats, not reshuffle the same handful of questions.

Common mistakes

  • Swapping complementary (90°) and supplementary (180°).
  • Forgetting a triangle's angles total 180°.
  • Arithmetic slips when subtracting from 90 or 180.
  • Calling an angle 'obtuse' when it's actually a straight (180°) angle, or vice versa.

Tips

  • Complementary = 90°, Supplementary = 180° (C comes before S, 90 before 180).
  • A triangle's angles always total 180°.

For parents

Complementary comes before Supplementary alphabetically, just like 90 comes before 180.

For teachers

Use a straight line and a right angle as visual anchors for supplementary and complementary.

Key vocabulary

acute
an angle less than 90°
obtuse
an angle between 90° and 180°
complementary
two angles that add to 90°
supplementary
two angles that add to 180°

Frequently asked questions

How do you classify an angle?
Acute is less than 90°, right is exactly 90°, obtuse is between 90° and 180°, and straight is exactly 180°.
What are complementary and supplementary angles?
Complementary angles add to 90°; supplementary angles add to 180°.
What do the angles of a triangle add up to?
Always 180°, so a missing angle is 180° minus the other two.

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